1. Field of the Invention
The present invention relates to switching power supply units, and more specifically, to a switching power supply unit that improves efficiency and performance in a light load operation.
2. Description of the Related Art
Switching power supply units related to the present invention are disclosed in U.S. Pat. Nos. 6,061,252 and 6,201,713.
FIG. 1 shows a diagram of a switching power supply unit disclosed in U.S. Pat. No. 6,061,252.
In the switching power supply unit, a transformer T having a primary winding T1 and a secondary winding T2, a first switching circuit S1, and an input power supply E are connected in series, and a series circuit of a second switching circuit S2 and a capacitor C is connected in parallel to the primary winding T1 of the transformer T. Furthermore, a rectifying and smoothing circuit including a rectifier element Ds is connected to the secondary winding T2 of the transformer T, and a capacitor Cs is connected in parallel to the rectifier element Ds. Furthermore, the transformer T includes a first drive winding T3 and a second drive winding T4. The first drive winding T3 is connected to a first control circuit 11 and the second drive winding T4 is connected to a second control circuit 12. The control circuits 11 and 12 which constitute a switching control circuit control the ON/OFF of the first and the second switching elements Q1 and Q2, respectively.
The first switching circuit S1 is defined by a parallel connection circuit including a first switching element Q1, a first diode D1, and a first capacitor C1. The second switching circuit S2 is defined by a parallel connection circuit including a second switching element Q2, a second diode D2, and a second capacitor C2. L indicates a leakage inductor in the primary winding T1 or an inductor that is connected separately.
In the configuration described above, the first and the second control circuits 11 and 12 constituting the switching control circuit control the first switching circuit S1 and the second switching circuit S2 to alternately turn the switching circuits S1 and S2 on and off with an intermediate period in which both of the switching circuits S1 and S2 are turned off, such that energy is stored in the primary winding T1 of the transformer T during an ON period of the first switching circuit S1 and the energy is discharged from the secondary winding T2 of the transformer T during an OFF period of the first switching circuit S1. The operation cycle is repeated such that energy is received from the secondary winding T2 and a power is supplied to the load. The first and the second control circuits 11 and 12 include transistors connected to the control terminals of the switching elements Q1 and Q2, and time constant circuits connected to the control terminals of the transistors, so that the circuits control when the switching elements Q1 and Q2 are turned on and off.
FIG. 2 is an operation waveform chart of the switching power supply unit. Referring to FIG. 2, Q1 and Q2 indicate when the switching elements Q1 and Q2 are turned on and off, respectively. Vds1 and Id1 indicate a voltage across and a current through the switching element Q1, respectively. Vds2 and Id2 indicate a voltage across and a current through the switching element Q2, respectively. Vs and Is indicate a voltage across the rectifier element Ds and a current through the secondary winding T2, respectively.
In the configuration described above, when the first switching element Q1 is turned off, a voltage is generated on the drive winding T4 for the second switching element Q2, whereby the second switching element Q2 is turned on. Then, the transistor in the control circuit 12 is turned on after a certain period of time determined by the time constant circuit has elapsed, whereby the second switching element Q2 is turned off. At this time, a voltage is generated in the drive winding T3 for the first switching element Q1 when the rectifier element Ds on the secondary side is turned off if the rectifier element Ds has been turned on, or when the second switching element Q2 is turned off if the rectifier element Ds has been turned off. That is, a voltage is generated when both of the second switching element Q2 and the rectifier element Ds are turned off, whereby the first switching element Q1 is turned on. By the operation described above, the first switching element Q1 and the second switching element Q2 are controlled to alternately turn on and off with an intermediate period in which both of the switching elements Q1 and Q2 are turned off, such that an energy which is stored on the primary winding of the transformer T during an ON period of the first switching element Q1 is output from the secondary winding T2 as electric energy during an OFF period of the first switching element Q1.
In the switching power supply unit described above, a zero-voltage switching operation is performed in which the first and the second switching elements Q1 and Q2 are turned on after voltages applied across each of the switching elements drops to zero. This reduces switching loss and prevents switching surge, to thereby improve efficiency and performance.
However, in the switching power supply unit described above, when the load is light, a portion of the energy stored in the primary winding T1 of the transformer T during an ON period of the first switching element Q1 is regenerated on the input side. In FIG. 3, regenerative current is shown as an area indicated by A. The regenerative current becomes a circulating current that does not contribute to output. If the ON period of the switching element Q2 is fixed, the circulating current increases as the output power decreases (as the load becomes lighter). A large circulating current greatly increases conduction loss in the first and the second switching elements Q1 and Q2 and the transformer T, resulting in greatly reduced efficiency under light load operation.
Accordingly, in a switching power supply unit according to U.S. Pat. No. 6,201,713, circulating current is reduced by shortening the ON period of the second switching element Q2 in a light load operation, thereby improving efficiency. However, a reduction in circulating current causes an increase in the switching frequency. This increase in switching frequency produces an increase in switching loss including loss in drive circuits for switching elements.
That is, if circulating current is large, switching loss is small because the switching frequency is low. However, conduction loss associated with the circulating current increases. On the other hand, if circulating current is reduced, conduction loss is reduced. However, switching loss increases due to the higher switching frequency.
To overcome the above-described problems, preferred embodiments of the present invention provide a switching power supply unit that reduces circulating current and thereby reduces conduction loss under light load operation or no load operation, and that also reduces switching loss and switching surge, thus improving the efficiency and reducing the size and weight of a switching power supply unit.
A preferred embodiment of the present invention provides a switching power supply unit including a first switching circuit including a parallel connection circuit that includes a first switching element, a first diode, and a first capacitor; a second switching circuit including a parallel connection circuit that includes a second switching element, a second diode, and a second capacitor; a capacitor connected in series to the second switching circuit, the series circuit of the capacitor and the second switching circuit are connected to one end of the first switching circuit; a transformer including a primary winding and a secondary winding; a rectifying and smoothing circuit including a rectifier element, connected to the secondary winding of the transformer; an input power supply connected in series to the first switching circuit and the transformer; and a switching control circuit which alternately turns the first switching element and the second switching element on and off with an intermediate period in which both of the switching elements are turned off, such that energy is stored in the primary winding of the transformer during an ON period of the first switching element and the energy is discharged from the secondary winding of the transformer during an OFF period of the first switching element; wherein the switching control circuit includes an OFF period extending device which permits an OFF period of the first switching element to be continued for a desired amount of time even after the energy has been discharged from the secondary winding, thereby lowering the switching frequency.
According to the prior art, when energy is discharged from the secondary winding, a voltage is generated in the drive winding for the first switching element when the rectifier element in the rectifying and smoothing circuit is turned off or when the second switching element is turned off. That is, the voltage is generated when both of the second switching element and the rectifier element are turned off, whereby the first switching element is turned on via the first control circuit.
According to the above-described preferred embodiment of the present invention invention, the OFF period extending device that permits the OFF period of the first switching element to be continued for the desired amount of time is provided. Thus, the first switching element is turned on with a delay, thereby lowering the switching frequency, whereby switching loss is reduced. Furthermore, the series circuit of the second switching element and the capacitor defines a voltage clamping circuit, thereby preventing a voltage surge across the first and the second switching elements. This allows for the use of low-voltage switching elements. Because low-voltage switching elements have small resistance values when turned on and are inexpensive, use of low-voltage switching elements greatly reduces loss, greatly improves efficiency, and greatly reduces cost.
The switching control circuit of another preferred embodiment of the present invention preferably includes a device for limiting an ON period of the second switching element to less than the time required to discharge the energy from the secondary winding.
By providing the device for limiting the ON period of the second switching element to less than the time required to discharge the energy from the secondary winding, i.e., a device for decreasing regenerative current, the discharge time of the capacitor connected in series to the second switching circuit is reduced in light load operation or no load operation. This reduces circulating current, and thereby greatly reduces conduction loss associated with the circulating current.
The OFF period extending device is defined by a transistor connected in series to the control terminal of the first switching element, the transistor remaining turned off even after the energy has been discharged from the secondary winding, such that the OFF period of the first switching element is extended for a desired amount of time.
For example, a device for detecting whether the load is light or heavy is provided for the secondary winding of the transformer, such that the transistor is controlled to remain turned off even after the energy has been discharged from the secondary winding if a light load is detected. According to this arrangement, a single transistor is required to define a switching element for driving the switching elements. Thus, the number of components is reduced, thus greatly reducing the size, weight and cost of the switching power supply unit.
Alternatively, the OFF period extending device may be defined by a transistor connected in parallel to the control terminal of the first switching element, the transistor remaining turned on even after the energy has been discharged from the secondary winding, such that the OFF period of the first switching element is extended for a desired time.
In contrast to the arrangement described above, in which the transistor is connected in series to the control terminal of the first switching element, in this arrangement, the transistor is connected in parallel to the control terminal of the first switching element, which also achieves the same advantages described above.
The switching control circuit preferably includes a transistor connected to the control terminal of the first switching element, and a time constant circuit defined by a capacitor and an impedance circuit connected to the control terminal of the transistor, such that the output voltage is controlled by controlling the ON period of the first switching element.
The time constant circuit is provided to turn on the transistor after a desired period of time elapses from an increase in voltage in the drive winding and to thereby quickly turn off the first switching element. The output voltage is easily controlled by adjusting the time constant of the time constant circuit, thus reducing the size, weight and cost of the switching power supply unit.
Furthermore, the switching control circuit preferably includes a transistor connected to the control terminal of the second switching element, and a time constant circuit including a capacitor and an impedance circuit connected to the control terminal of the transistor, such that the discharge current from the capacitor connected in series to the second switching circuit is controlled by controlling the ON period of the second switching element.
In this arrangement as well, similar to the arrangement described above, the output voltage is controlled by adjusting the time constant of the time constant circuit, thus reducing the size, weight and cost of the switching power supply unit.
The OFF period extending device preferably operates in a light load operation, and does not operate in at least a heavy load operation.
The OFF period extending device produces optimal operation in accordance with the load when it operates in a light load operation in response to a signal indicating detection of the light load operation and does not operate in at least a heavy load operation. This achieves highly efficient operation in no load, light load, and heavy load operations.
The first switching element and the second switching element preferably include field-effect transistors.
By constructing the first and the second switching elements using field-effect transistors, parasitic capacitances of the field-effect transistors are used as the first capacitor and the second capacitor, and parasitic diodes of the field-effect transistors are used as the first diode and the second diode. This eliminates the need to provide the diodes and the capacitors as separate components, thus further reducing the size, weight and cost of the switching power supply unit.
The transformer preferably includes a first drive winding and a second drive winding for generating voltages that turn on the first and the second switching elements to produce a self-excited oscillation.
Because the drive windings are provided in the transformer to produce a self-excited oscillation, ICs such as an oscillation circuit and a control circuit are not required, thus further reducing the size, weight and cost of the switching power supply unit.
The switching power supply unit may further include a leakage inductor induced in a magnetic circuit including the primary winding and the secondary winding of the transformer or an inductor connected in series to the transformer, wherein the inductor and the capacitor connected in series to the second switching element define an oscillation circuit.
The capacitor and the leakage inductor in the transformer or the inductor separately connected define an oscillation circuit that generates oscillation. Thus, energy stored in the inductor is output to avoid loss, and improve efficiency. In addition, the second switching element is turned off with a zero current to reduce switching loss.
Preferably, the rectifying and smoothing circuit includes a diode as the rectifier element, and a capacitive impedance is connected in parallel to the diode.
By connecting the capacitive impedance to the rectifier element, the reverse recovery loss of the rectifier is greatly reduced and efficiency is greatly improved. In addition, energy transmitted to the secondary winding is supplied to the load via the capacitive impedance without passing the rectifier element, so that rectification loss is reduced. Furthermore, the capacitance of the first capacitor and the second capacitor in the first switching circuit and the second switching circuit respectively, is greatly reduced.
Other features, elements, steps, characteristics and advantages of the present invention will become apparent from the following detailed description of preferred embodiments with reference to the attached drawings.